1. Field of the Invention
The present invention relates to a Thin Film Transistor-Liquid Crystal Display (TFT-LCD), and more particularly, to a structure and method for laser repair to enhance the display quality.
2. Description of the Prior Art
Generally, a TFT-LCD panel has a liquid crystal layer sandwiched by two substrates with electrodes thereof. The light transmittance of the LCD can be controlled by applying voltages to the respect electrodes.
The display region of a TFT panel contains a plurality of pixels arranged in a matrix type. A pixel is a rectangle region defined by the crossing of two gate lines and two data lines. In addition to a pixel electrode, there are a TFT element and a storage capacitor contained in the pixel. The TFT is a switching element in which the on-off state is controlled by both the scanning signals from the gate line and the data signals from the data line. A storage capacitance line is provided in the pixel to form a capacitor that can maintain the present pixel signal of the pixel electrode until the subsequent signal is applied thereto.
It is known that the TFT-LCD has been developed towards the television application in recent years, so the display panel has been promoted to the large-scale design. Consequently, the fabrication process is more and more complex and difficult. It is hard to consider the influences of both the constraint conditions and the process errors on the display quality at the same time for the TFT-LCD design, although they are the key factors to affect the productivity and the yield.
During the procedures of fabricating the TFT panel, the pixel is liable to be damaged by several factors such as the unexpected particle pollution and the static electricity. A line defect means a broken signal line, and a pixel defect means the abnormal short or open circuits of TFT.
The pixel defects can be distinguished as several kinds such as the white defect, the black defect, and the dull defect, etc. Since the white defect is always bright that is very easy to be recognized by human's eyes, it is preferable that the white defect is repaired to be a black defect that is always dark. Or at least, the white defect needs to be shifted into a dull defect that is always slightly bright.
Generally, a laser repair method will be executed when just a few white defects occur.
A top-view schematic diagram of a pixel with the conventional laser repair structure in the TFT panel is illustrated in FIG. 1. Data line 114 transmits the data signal to the source electrode 100, and the gate signal is transmitted by the gate line 104 that is located in the gate-electrode layer on the upper surface of a transparent glass substrate. A storage capacitance line 110, which is located in the gate-electrode layer, is provided in the pixel to transmit the common voltage. The semiconductor electrode 102 is partially covered by the source electrode 100 and the drain electrode 106, and the contact hole 108 is to electrically connect the pixel electrode 112 and the drain electrode 106. One additional metal conductor 116, which is located in the source-drain layer under the pixel electrode 112 and is electrically connected to the pixel electrode 112 through the contact hole 120, is prepared for the required laser repair. Another additional metal conductor 118, which is located in the gate-electrode layer and is normally floating, is also prepared for the required laser repair. The metal conductor 118 is partially overlapped with the data line 114 and the metal conductor 116 at the portions 124 and 122 separately.
Once the white defect of the pixel has been found, the laser beams can be used to irradiate the overlapped portions 124 and 122 from the lower surface side of the transparent glass substrate to electrically connect the data line 114 and the metal conductor 116. Then, the data signal can directly transmit to the pixel electrode 112 through the contact hole 120 so as to convert the white defect into a dull defect.
However, this laser repair structure for repairing white defect needs an additional metal conductor with a contact hole located in the source-drain layer under the pixel electrode. Thus, the additional metal conductor will decrease the aperture ratio of the pixel.
Except for the pixel defect of the TFT panel, line defect is also happened in the manufacturing procedures. It needs to be repaired, too.
FIG. 2 is a top-view schematic diagram of a pixel in the TFT panel, which shows a conventional laser repair structure for a pixel having an open-circuited portion 216 in the data line 214. The gate signal is transmitted by the gate line 204 that is located in the gate-electrode layer on the upper surface of a transparent glass substrate. Because the data line 214 has an open-circuited portion 216, it fails to transmit the data signal to all the source electrodes connected to it including the source electrode 200 of this pixel. A storage capacitance line 210, which is located in the gate-electrode layer, is provided in the pixel to transmit the common voltage. The semiconductor electrode 202 is partially covered by the source electrode 200 and the drain electrode 206, and the contact hole 208 is to electrically connect the pixel electrode 212 and the drain electrode 206. Two repair members 218, 220 are extended from the storage capacitance line 210 and overlapped with the data line 214 at the extending portions 222, 224 separately, which are prepared for the required laser repair.
Once an open-circuited portion 216 of the data line 214 has been found, the laser beams can be used to irradiate the overlapped portions 222 and 224 from the lower surface side of the transparent glass substrate to electrically connect the data line 214 and the storage capacitance line 210. Then, using a low-energy laser to irradiate the location 226 to evaporate some portion of the pixel electrode 212. Thereafter, using a high-energy laser to irradiate the location 226 to evaporate some portion of the storage capacitance line 210 until the storage capacitance line 210 is open circuit in the location 226. Therefore, the data line 214 is repaired by using some portion of the storage capacitance line 210 and the data signal can be transmitted to all the source electrodes connected to it.
However, it is not easy to control in melting the pixel electrode by the laser because the pixel electrode is normally constituted by the indium tin oxide (ITO) or the indium zinc oxide (IZO). This kind of laser repair structure and method for line defect needs to use a low-energy laser to evaporate some portion of the pixel electrode, so it's very hard to control.
Consequently, such a laser repair structure and method for line defect is very complicated and risky. Furthermore, it is easy to cause a bad short-circuit between the data line and the storage capacitance line when doing the laser repairing because it use the storage capacitance line as a lead, and so as to result in a low repair yield.